Chemical sensors based on surface acoustic wave (SAW) filters are known. A typical SAW filter consists of two interlaced comb-like metal structures, referred to as interdigitated transducers (IDTs), which are deposited on a piezoelectric crystal surface. The first IDT serves as a transducer to convert long wavelength radio waves to very short wavelength acoustic waves. The second IDT converts the acoustic waves back into an electromagnetic signal. By way of example, 3 GHz radio waves propagating in free space have wavelengths of 10 cm, whereas 3 GHz acoustic waves propagating in Y-Z lithium niobate, an illustrative piezoelectric material, have wavelengths of only 0.000116 cm.
Electrically, a SAW filter behaves as a band-pass filter. That is, it has very low insertion loss in a portion of the frequency spectrum (i.e. the pass band), and it has very high insertion loss in another portion of the spectrum (i.e. the stop band).
In operation, an alternating voltage, typically at microwave frequency, is applied across the first, transmitting, IDT. Because the substrate is piezoelectric, the applied voltage induces a mechanical distortion on the surface of the piezoelectric material which propagates preferentially along the surface of the crystal as an acoustic wave. Proper orientation of the underlying substrate can enable this wave to propagate as a surface-confined Rayleigh wave with little loss or dispersion. The addition of the second, receiving, second IDT creates a two-port electrical device which can be designed to respond electrically with the desired band pass configuration.
As is well-known in the art, confinement of the Rayleigh wave's energy into the thin surface region of the SAW device enables the creation of a sensitive mass detector. The basic principle of operation of such a detector is that mass loading due to chemical species adsorbed typically into the chemical coating on the substrate surface in the acoustic propagation path will cause a measurable change the propagation characteristics of the acoustic wave, typically observable as a phase delay, relative to a similar wave in a pristine reference device.
Accordingly, a measurement system will typically include two SAWs, i.e. a measurement and a reference device. These devices are substantially identical SAWs kept in close physical proximity under substantially identical conditions of temperature and exposure to the gas being measured. Both of these SAWs must be designed in a manner to provide a physical separation region between the two IDTs. The shape and size of this space between the IDTs along with the dimensions of the IDTs themselves largely determines the physical dimensions of the SAW. Additionally, the separation length between the IDTs affects the magnitude of the measurable phase delay and the insertion loss of the entire SAW. The width of the propagation path, which we refer to herein as the “aperture”, is equal to the width of the IDTs.
For detection, an analyte is exemplarily injected into a stream of carrier gas, which then passes over the detector surface. For environmental sensing, a sample of the ambient atmosphere which may contain an analyte of interest may likewise be directed over the detector surface.
Although in some cases the bare substrate surface may adsorb sufficient analyte molecules to provide a measurable signal, it is more common to increase sensitivity by applying a sensitizing chemical coating to the surface of the substrate in the propagation path between the IDTs. A coating may be used that is an effective adsorbant for a general variety of chemical species, or a chemically selective adsorbant may be used. In the following discussion and claims, we will use the term “adsorbant” to refer to the sensitizing layer because adsorption is the most common mechanism for the retention of analyte molecules. However, we do not mean to exclude other possible sorption mechanisms such as absorption and chemisorption. Hence, the terms “adsorption”, “adsorbant”, and “adsorptive” should be understood broadly as encompassing all forms and mechanisms of sorption.
SAW-based chemical detectors have proven to be useful for numerous applications. However, there remains a need for detectors of still greater sensitivity.